The present invention relates to semiconductor structures with deep localized grid accessible via the surface and a process for manufacturing same.
In different types of discrete or integrated semiconductor components, such as high-power bipolar transistors, gate turn-off thyristors, junction power vertical field-effect transistors and others, there is a deep localized layer in the form of a grid disposed inside the semiconductor wafer in a plane parallel to that of the principal faces of this wafer.
Such deep layers are currently designated in the technique by the expression buried grids because of the conventional manufacturing process of the prior art by which they were obtained. In fact, starting from a semiconductor substrate, this process included implanting therein dopant atoms according to the type of conductivity corresponding to that of the grid which it was desired to obtain, then in forming above this substrate one or more epitaxial layers in which one or more diffusions are possibly formed. Such a structure is shown schematically in the accompanying FIG. 1. There can be seen therein the substrate 1 overlaid by an epitaxial layer 2 in which is formed a diffused zone 3. The buried layer or grid 4 is present at the limit between substrate 1 and the epitaxial layer 2. Then, a contact is provided between the upper surface of the semiconductor wafer and the buried grid 4 by means of a deep diffusion 5. In numerous practical applications, grid 4 must have an extremely fine and well-defined pitch. Furthermore, this grid serves generally for turning off the semiconductor device. It is then necessary for the transverse resistivity of the grid layer to be as low as possible. Its doping level must then be very high. The high doping level makes it difficult to control with great accuracy the extent of the diffusion from the initially implanted layer so that the mesh of the grid do not close and have a predetermined thickness. The control of this diffusion is very delicate to achieve particularly because of the other thermal diffusion steps which must be provided for the formation of the other layers formed on the substrate. Parasite diffusions of the implanted dopant may also occur during formation of the epitaxial layer.
These drawbacks are all the more noticeable since, in numerous devices, attempts have been made to obtain in a practical way grid mesh dimensions of the order of a few microns. Moreover, because of the very fact of using an epitaxial layer deposited on a substrate, there cannot develop between the mesh of the grid, at the level of the interface between the epitaxial layer and the substrate, a space charge required for the proper operation of the device.
Thus, an object of the present invention is to provide a novel deep-grid semiconductor device structure which palliates the structural or manufacturing drawbacks of the buried-grid devices of the prior art.
Another object of the present invention is to provide a novel process for manufacturing a deep-grid semiconductor device.
To attain these objects as well as others, the present invention provides a deep-grid semiconductor device structure not comprising a layer buried under an epitaxial layer in accordance with the technique which has been adopted during these latter years.